Static minimum excitation system for alternators



1954 s. 1.. BRADLEY ETAL 2,666,885

STATIC MINIMUM EXCITATION SYSTEM FOR ALTERNATORS Filed Aug. 9, 1952 2 Sheets-Sheet l INVENTORS WITNESSES:

1954 s. 1.. BRADLEY ETAL STATIC MINIMUM EXCITATION SYSTEM FOR ALTERNATORS Filed Aug. 9, 1952 2 Sheets-Sheet 2 Fig.2.

Voltage Across Field Winding 32 Fig.3.

2! 3,5220; 2 mourn; :55

Output Voltage of Rectifier I30 INVENTORS WlTNESSES:

Patented Jan. 19, 1954 STATIC MINIMUM EXCITATION SYSTEM FOR ALTERNATORS Schuyler L. Bradley, Pittsburgh, Pa., and Neil Nichols, Southgate, Calif., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application August 9, 1952, Serial No. 303,578

6 Claims.

This invention relates generally to regulating systems and more particularly to regulating systems incorporating means for preventing the excitation voltage of a regulated machine from decreasing below a predetermined value.

Heretofore many systems have been utilized for preventing the field excitation of a regulated machine from decreasing below a predetermined value. However, these prior art systems have many limitations. For instance, some of them do not respond to negative minimum excitation voltage settings and oftentimes the regulated voltage is affected by excitation changes over the normal range. 'In addition, in these prior art systems the intelligence portion of the circuit draws a large amount of energy from the potential source and requires a high level for the damping of the circuit. Further many of the circuits hereinbefore utilized have electronic tubes incorporated therein which require a considerable amount of maintenance. Also, these prior art systems are difiicult to adjust and set accurately.

An object of this invention is to provide for so biasing a minimum excitation system for a regulated machine that the system is polarized causing it to respond to either positive or negative minimum excitation voltage settings.

Another object of this invention is to provide for preventing an output from a minimum excitation system until the minimum excitation level is reached, by utilizing an amplifier whose output is made unidirectional and then So biased as to have no output until the minimum excitation level is reached, whereby the regulated voltage is not afiected by excitation changes over the normal range.

A further object of this invention is to provide for readily and accurately adjusting a minimum excitation system by properly disposing a magnetic amplifier in the system, the gain of which is readily adjustable.

A still further object of this invention is to provide for minimizing the amount of energy drawn from a potential transformer which supplies the voltage reference network and the minimum excitation system, by properly disposing a magnetic amplifier in the system.

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which Figure 1 is a schematic diagram illustrating an embodiment of the teaching of this invention;

Fig. 2 is a graph illustrating the manner in which the excitation voltage of the regulated machine varies with changes in the voltage across the auxiliary control or field winding of the rotary amplifier incorporated within the system, and

Fig. 3 is a graph illustrating the manner in which the output voltage of the sensing network incorporated in the minimum excitation system varies with its alternating current input voltage.

Referring to Fig. 1 of the drawing, there is illustrated an embodiment of the teaching of this invention in which a synchronous condenser It, comprising a main field winding [2, is disposed to supply energy to load conductors l4, l6 and It. In order to control the magnitude of the voltage across the field winding l2, a main exciter 22 is provided. In this instance, the main exciter 22 comprises an armature 24 which is electrically connected across the field winding I2 and a shunt field winding 26, the magnitude of the voltage across which is adjusted by means of a variable resistor 21.

For the purpose of controlling the output of the main exciter 22, a rotary amplifier 23 is provided. As illustrated, the rotary amplifier 28 comprises an armature 30, an auxiliary field or control winding 32, a main field or control winding 3'4, and a series field winding 36. Of course it is to be understood that any suitable magnetic amplifier or electronic amplifier could be substituted for the rotary amplifier 28 by one skilled in the regulator art, the particular amplifier used being responsive to the voltages that appear across the windings 32 and 34. In this instance, a field winding 31 is inductively coupled with the shunt field winding 26 of the main exciter Z2 and it is responsive to the output of the rotary amplifier 28. In operation, the series field wind ing 36 supplies the majority of the excitation requirements for the rotary amplifier 28, the remainder of the excitation being supplied by the field Winding 34 and the auxiliary field winding 32.

In order to maintain the output voltage of the synchronous condenser l 0 substantially constant, a voltage reference network 38 is provided. The voltage reference network 38 comprises a nonlinear impedance circuit 40 and a linear impedance circuit 42 connected to be simultaneously energized in accordance with the output voltage of the synchronous condenser In. As illustrated, the circuits 40 and 42 are electrically connected to the load conductors l6 and I8 through a variable potential transformer 44. The non-linear impedance circuit 40 includes a saturating reactor 48, whereas the linear impedance circuit :12

includes a capacitor 50. The saturating reactor 23 and the capacitor 56 have intersecting impedance characteristics so that when the output voltage of the synchronous condenser I is above the regulated value, the saturating reactor 48 draws more current, and when it is below the regulated value the capacitor 56 draws more current.

The non-linear impedance circuit 40 is electrically connected across a dry type rectifier unit 52 and the linear impedance circuit 42 is connected across the dry type rectifier unit 54 through an isolating transformer 56. In this instance, the output terminals of the rectifier units 52 and 54 are connected in series circuit relation ship with each other, the terminal 60 of the rectifier 52 being connected through series connected resistors 62 and 64 to the terminal 66 of the rectifier 54, and the terminal of the rectifier 52 being connected through the series connected smoothing reactors I2 and E4 to the terminal It or the rectifier 54.

In order to control the magnitude of the voltage across the field winding 34 of the rotary amplifier 29, in accordance with the output voltage of the synchronous condenser I0, one side of the field winding 34 is electrically connected to the junction point of the smoothing reactors I2 and i4 and the other side of the field winding 34 is electrically connected to the junction point of the resistors 62 and 64. Thus, if the output voltage of the synchronous condenser I0 is above the regulated value, current will flow in a predetermined direction through the field winding 34. However, if the output voltage is below the regulated value, current will flow in the opposite direction through the field winding 34.

The minimum excitation system illustrating this invention comprises a sensing network 60 which is responsive to the voltage across the field winding I2 of the synchronous condenser I0, a push-pull magnetic amplifier 82, which is responsive to the output of the sensing network 60, a source of biasing voltage 84 for the sensing network t6 and the magnetic amplifier 82, and a rectifier 36 connected to the output of the mag netic amplifier 82 for preventing the flow of current in one direction from the output of the amplifier 62. It is of course to be understood that a suitable electronic amplifier could be substituted for the magnetic amplifier 82, however, some of the advantages obtained by utilizing a magnetic amplifier in the system would not be ob tained by utilizing an electronic amplifier.

In particular, the sensing network 86 comprises a saturable reactor 90 for amplifying the signal received from across the field winding I2 of the synchronous condenser I0 and for polarizing the system. The sensing network 80 also comprises a resistance, capacitance and inductance network 92 for producing at the output of the sensing network 30 a voltage whose curve shape, once it is amplified by the magnetic amplifier 82, is substantially identical to the voltage curve representing the variable voltage across the field winding 34 0f the rotary amplifier 28 when the output of the synchronous condenser I0 is above its regulated value. In this instance, the saturable reactor 90 comprises two magnetic core members 94 and 96 which have disposed in inductive relationship therewith control windings 99 and I00, respectively. In order that the current flow through the control windings 98 and E00 is proportional to the magnitude of the voltage across the field winding I 2 of the synchronous condenser I 0, the control windings 98 and 200 art connecteq in series circuit relationship with one another and across the field winding I2. As illustrated, a variable resistor I02 is likewise connected in series circuit relationship with the control windings 96 and I00 so that the magnitude of the current flow through the control windings 38 and I00 can be readily varied.

The source of biasing voltage 84 comprises a full wave dry type rectifier I04 which receives energy from a potential transformer I06 which has a primary winding I08 and two secondary winding sections H0 and H2. In particular, the input terminals of the rectifier I04 are electrically connected across the secondary winding section II2 of the transformer I06. However, the output terminals of the rectifier I 04 are electrically connected to two biasing windings H6 and H8 which are connected in series circuit relationship with one another and with a variable resistor II9, the windings H6 and H6 being disposed in inductive relationship with the core members 94 and 96, respectively. As can be seen from the drawing, the control winding 96 and the biasing winding II6 are so disposed on the core member 94 that the current flow through them produces a flux in the core member 94 in the same direction. In like manner, the control winding I00 and the biasing winding II8 are so disposed on the core member 96 that the current flow through them produces a flux in the same direction in the core member 96.

In this instance the network 92 comprises a variable resistor I26, an inductance I24, and capacitors I25 and I26. As illustrated, the capacitor I25 and the inductance I24 are connected in parallel circuit relationship with one another, the parallel circuit being connected in series circuit relationship with the capacitor I 26. In order to rectify the output current of the sensing network 80, a full wave dry type rectifier I30 is provided. One of the input terminals of the rectifier I30 is electrically connected to one side of the parallel circuit comprising the capacitor I25 and the inductance I24.

In order to apply the voltage appearing across the secondary win-ding section H0 of the transformer I06 to the combination of the variable resistor I 29, the capacitor I26, the parallel circuit comprising the capacitor I25 and the inductance I24, and the rectifier I30, one side of the secondary winding section H0 is electrically connected to the other input terminal of the rectifier I30 and the other side of the winding III] is electrically connected to one side of the variable resistor I20, the other side of the resistor I 29 being connected to the capacitor I 26.

In order to vary the voltage at the output terminals of the rectifier I30 and thus the output voltage of the sensing network 86, main windings I34 and I36 of the saturable reactor 90 are disposed in inductive relationship with the core members 94 and 96, respectively. As illustrated, the main windings I34 and I 36 are connected in series circuit relationship, one end of the winding I36 being connected to one of the input terminals of the rectifier I30 and one end of the winding I34 being connected to the junction point of the variable resistor I20 and the capacitor I26.

As hereinbefore mentioned, the sensing network produces at its output a voltage which increases in a predetermined manner. However, the shape of the curve representing the manner in which the output voltage of the sensing network 80 increases can .be changed by adjusting the variable resistor I20 and also by adjusting a 5. variable resistor Hi5 which is electrically connected between the sensing network 30 and the magnetic amplifier 82.

The push-pull magnetic amplifier B2 amplifies the output of the sensing network 8i and under one set of conditions as explained hereinafter cooperates with the rectifier 85 to permit current to flow only in one direction from the output of the amplifier t2 and then only when the output from the sensing network at has reached a predetermined value. In this instance, the magnetic amplifier 22 comprises a plurality of magnetic core members Mt, i455, E43 and 156. In order to vary the saturation of the core members M l, M5, M8 and IE6 in accordance with the output voltage of the sensing network a plurality of control windings I52, i5 3, H53 and IE5 are disposed in inductive relationship with the core members HM, I48, I48 and [5G, respectively. As

illustrated, the control windings l52, [5d, I56 and E58 are connected in series circuit relationship with the variable resistor Mil, the series circuit being connected across the output terminals of the rectifier Hie.

A plurality of main or reactor windings I60, H52, IM and 565 are likewise disposed in inductive relationship with the core members use, l te, M8 and Hill, respectively. A transformer iii? having a primary winding I'EZ connected to a suitable alternating current source ill!- and sec ondary winding sections lit and H8 is provided in order to supply energy to the reactor windings its, let, its and H36. In order that current flows in only one direction through the reactor windings i622 and through the reactor windings Hi l and lfit, and in order to obtain a direct current voltage at the output of the magnetic amplifier 32, dry type full wave rectifiers i 8%? and H32, respectively, are provided. As illustrated, the reactor windings lite and W2 ar connected in series circuit relationship, one end of this series circuit being connected to one end of the secondary winding section lie, the other end of the circuit being connected to one of the input terminals of the rectifier let. The other input ter" minal of the rectifier [St is connected to the other end of the winding section l'iii. In like manner the reactor windings H34 and let are connected in series circuit relationship, one end of this series circuit being connected to one end at the secondary winding section lit, and the other end of the circuit being connected to one of the input terminals of the rectifier 582. The other input terminal of the rectifier i872 is connected to the other e or" the winding section H2.

In. order to provide both a feedback and a biasing current for the core members, windings I84, ltii, let and tilt are disposed in inductive relationship with the core members M 1 Mil, Hit and sec, respectively. The windings led and use are connected in series circuit relationship, one end of this series circuit being connected to one of the output terminals of the rectifier ace and the other end of the series circuit being connected to the other output terminal of the rectifier Hit through a portion of a variable resistor 92 and a variable resistor IN. In like manner, the windings i823 and use are connected in series circuit relationship with one another, one end of this series circuit being connected to one of the output terminals of the rectifier E82 and the other end being connected to the other output termi" nal of the rectifier Hi2 through another portion of the variable resistor H92 and a variable resistor I96.

It is to be noted that the minimum excitation system of this invention can readily be adjusted by means of the variable resistors I92, i9 0, and 95, that is the output of the amplifier 532 can be readily varied, thus changing the slope of its output curve, and a proper balance of the push-pull magnetic amplifier 82 is also readily obtained.

In this embodiment the current fiow through the control winding I52 and through the biasfeedback winding i534 produces a fiux in the core member [M in the same direction. In like mannor, th current flow through the control winding I54 and through the bias-feedback winding I86 produces flux in the same direction in the core member I46. However, the current flow through the control winding E56 and through the biasfeedback winding 133 produce fluxes which oppose one another in the core member 48. In like manner, the current flow through the control winding !58 and through the bias-feedback winding I99 produces fluxes which oppose one another in the core member l In order to bias the core members i5 5, I46, 548 and E59 so that the output from the rectifier 152 is greater than the output from the rectifier iiiil when the output voltage of the synchronous condenser H) is at its regulated value, biasing windings 2G0, 202, 2% and 206 are properly disposed in inductive relationship with the core members Hit, E45, M3, and. E58, respectively. In this instance, the biasing windings 200, 202, 2% and 2% are connected in series circuit relationship with one another, one end of the series circuit being connected through a variable resistor its to one of the output terminals of the rectifier le t and the other end of the series circuit being connected through a switch 2m to the other output terminal or the rectifier N14. The purpose of the variable resistor 268 is to provide means for varying the current fiow through the biasing windings 2%, 202, 206 and 295. The purpose of the switch 2 i ii will be described hereinafter. As can be seen from the drawing, the biasing winding res and the bias-feedback winding 8 are so disposed on the core member Md as to produce fluxes therein that oppose one another. Likewise, the biasing winding 2&2 and the bias-feedback winding Hit are so disposed on the core member 545 as to produce fluxes therein that oppose one another. On the other hand, the biasing winding 2% and the bias-feedback I88 are so disposed on the core member its as to produce fluxes therein that are in the same direction. Likewise, the biasing winding 29% and bias-feedback I98 are disposed on the core member E56 as to produce fluxes therein that are in the same direction.

In order to render the auxiliary winding 32 of the rotary amplifier 28 responsive to the output voltage of the magnetic amplifier 82, one side of the winding 32 is electrically connected to one of the output terminals of the rectifier E82 and the other side of the winding 32 is electrically connected to one of the output terminals of the rectifier let through the rectifier $35. A switch 216 is electrically connected across the rectifier 85, the purpose of which will be described hereinafter.

During normal operating conditions, the voltage reference network 38 in conjunction with the rotary amplifier 28 and the main exciter 22 maintains the output voltage of the synchronous condenser l0 substantially constant. However, assuming the output voltage of the synchronous condenser It increases to such as value as to tend to drive the excitation voltage across the field winding I2 of the synchronous condenser I below a safe operating value, where the synchronous condenser I0 would fall out of step, the sensing network 80 and the magnetic amplifier 82 function to prevent the voltage across the field winding I2 from decreasing below a safe operating point.

Assuming the switch 2 I 0 is open and the switch 2I6 is closed, the apparatus operates as follows. The increased output voltage of the synchronous condenser I0 above the regulated value applied to the voltage reference network 33 effects a decrease in the magnitude of the voltage across the field winding I2. With a decrease in voltage across the field winding I2 the current fiow through the control windings 93 and I00 of the saturable reactor 90 decreases to thereby decrease the saturation of the core members 94 and 96, respectively, and thus increase the impedance of the reactor windings I34 and I36, respectively. Such an action decreases the voltage drop across the variable resistor I and thereby increases the voltage across the capacitors I and I 23 and the inductance I24 and thus the voltage across the input terminals of the rectifier I30. This, in turn, increases the output voltage of the rectifier I as well as the magnitude of the current flow through the control windings I52, I54, I55, and I58 or" the magnetic amplifier 32.

Since the current flow through the control winding I52 and the bias-feedback winding I84 and through the control winding I54 and the bias-feedback winding I89 produce fiuxes that are additive in the core members I 14 and I46, respectively, the saturation of these core members is increased to thereby increase the current fiow through the reactor windings I60 and IE2 and thus increase the voltage across the input terminals of the rectifier I 80. However, since the current fiow through the control winding I59 and through the bias-feedback winding I88 and through the control winding I58 and the biasfeedback winding I90 produce fluxes which oppose one another in the core members I48 and I50, respectively, the saturation of these core members is decreased. This, in turn, increases the impedance of the reactor windings IE4 and I66 to thereby decrease the magnitude of the voltage across the input terminals of the rectifier I 82 and thus the voltage across its output terminals.

Since the variable resistor I 92 is connected between one of the output terminals of the rectifier I30 and one of the output terminals of the rectifier I 82 and the series connected variable resistors I94 and I 99 are connected between the other output terminals of the rectifiers I 80 and I82, there being an electrical connection between the junction point of the resistors I94 and I96 and the mid-section of the resistor I92, current will flow out of one of the output terminals of the rectifier I 82 through the auxiliary field winding 32 and the closed switch 2 I B to one of the output terminals of the rectifier I80. As hereinbefore mentioned, the sensing network 80 in conjunction with the magnetic amplifier 82 produces at the output of the amplifier 82 a voltage that increases in substantially the same manner as the voltage across the field winding 34 of the rotary amplifier 28 increases to thereby prevent the voltage across the field winding I2 of the synchronous condenser I0 from decreasing below a predetermined value.

When the switch 2I0 is open and the switch H6 is closed, the field excitation voltage across the field winding I 2 does not level off immediately to the minimum excitation value, but rather decreases gradually at first, as represented by the first portion of curve A of Fig. 2, to the given minimum excitation value. This gradual decrease at first represented by curve A is caused by the gradual increase at first of the output voltage of the sensing network 80, as represented by curve B, Fig. 3. There is no output from the sensing network 89 and thus from the rectifier I36 until point C, as illustrated in Fig. 3, is reached. Since the minimum excitation level for the synchronous condenser I0 is not reached until the output voltage of the rectifier I30 increases to the point D, there is a period of time between the points C and D in which the regulated voltage of the synchronous condenser I0 is affected.

In order to overcome the above objections, the switch 2I0 is closed and the switch 2I0 is open. Under these conditions there is no output current from the magnetic amplifier 92 until the point D on curve B is reached. Thus, the voltage across the field winding I2 of the synchronous condenser I0 decreases to the predetermined minimum excitation value and at that point levels off immediately as represented by the horizontal curve E of Fig. 2, thus preventing the regulated voltage from being aiiected over the normal range. This action is afiected by the bias Windings 200, 202, 204 and 296 which, as hereinb-efore mentioned, are so disposed on their respective core members as to prevent a current flow out of the rectifier I82 to the auxiliary field winding 32 until the point D on curve B is reached, the rectifier 8E preventing the flow of current in the opposite direction through the auxiliary winding 32.

The apparatus illustrating the teachings of this invention has several advantages. For instance, it employs a biasing scheme which polarr'es the unit so that it may respond to both positive and negative minimum excitation voltage settings. Further, by using a push-pull magnetic amplifier whose output is made unidirectional, and then biased so as to have no output until the minimum excitation level is reached, the regulated voltage is not affected by excitation changes over the normal range. The use of the magnetic amplifier also allows the intelligence portion of the circuit to draw a much lower amount of energy from the potential source and it requires a lower level for damping of the circuit. In addition, maintenance costs are low and dependability high, particularly when no electronic tubes are incorporated into the apparatus.

In accordance with the teachings of this invention an absolute minimum excitation value may be set, instead of a minimum value proportional to the rise in alternating-current voltage. With the magnetic amplifier whose gain is adjustable, the system can be set more accurately and with greater ease.

Since certain changes may be made in the above-described apparatus, and different embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. In a system for preventing a voltage regulator from driving the excitation level of a regulated machine below a safe predetermined minimum value, the combination comprising, a field wind- 9 ing for the regulated machine, an amplifier comprising two control windings and disposed to control the energy supplied to the field winding of the regulated machine, a voltage reference network responsive to the output of the regulated machine, the output of the voltage reference network being electrically connected to one of the control windings of said amplifier to produce thereacross a voltage which increases in a predetermined manner, a sensing network responsive to the voltage across the field winding of the regulated machine and which produces at its output a Voltage which rises in a predetermined manner, and another amplifier responsive to the output of the sensing network, the output of said another amplifier being electrically connected to the other of the control windings of said amplifier, whereby the voltage across said other of the control windings of said amplifier after reaching a predetermined value is equal to the voltage appearing across one of the control windings of said amplifier and increases in the same predetermined manner as the voltage across said one of the control windings of said amplifier to thereby prevent the voltage across the field winding of the regulated machine from decreasing below the predetermined minimum value.

2. In a system for preventing the field excitation of a regulated machine from going below a safe predetermined minimum value, the combination comprising, a field winding for the regulated machine, an amplifier comprising two control windings and disposed to control the energy supplied to the field winding of the regulated machine, a voltage reference network responsive to the output of the regulated machine, the output of the voltage reference network being electrically connected to one of the control windings of said amplifier, whereby a voltage is produced across said one of the control windings of said amplifier which increases in a predetermined manner, a sensing network comprising a saturable reactor and an impedance network the saturable reactor being responsive to the voltage across the field winding of the regulated machine and said network being responsive to the output of the saturable reactor to thereby produce at its output a voltage which increases in a predetermined manner, and another amplifier responsive to the output of the sensing network, the output of said another amplifier being electrically connected to the other control winding of said amplifier, whereby the voltage across said other control winding of said amplifier after reaching a predetermined value is equal to the voltage appearing across said one of the control windings of said amplifier and increases in the same predetermined manner as the voltage across said one of the control windings of said amplifier, to thereby prevent the voltage across the field winding of the regulated machine from decreasing below the predetermined minimum value.

3. In a system for preventing a voltage regulator from driving the excitation level of a regulated machine below a safe predetermined minimum value, the combination comprising, a field winding for the regulated machine, an amplifier comprising two control windings and disposed to control the energy supplied to the field winding of the regulated machine, a voltage reference network responsive to the output of the regulated machine, the output of the voltage reference network being connected to one of the two control windings of said amplifier to produce thereacross a voltage which increases in a predetermined manner, a sensing network responsive to the voltage across the field winding of the regulated machine and comprising a saturable reactor and an impedance network, the saturable reactor comprising a magnetic core member having a control winding disposed in inductive relationship therewith, the current flow through which is proportional to the voltage across the field winding of the regulated machine, a source of biasing voltage, a biasing winding disposed in inductive relationship with the core member and responsive to the output of said source, a main winding disposed in inductive relationship with the core member, a rectifier having input and output terminals, one of the input terminals being connected to one side of the impedance network, an alternating current source of supply connected to the other input terminal of the rectifier and to the other side of said impedance network, circuit means for connecting said main winding to said impedance network and to said other input terminal of the rectifier, whereby the impedance of the main winding varies in accordance with. the voltage across the field winding of the regulated machine and the voltage across the output terminals of the rectifier is proportional to the impedance of the main winding, and another amplifier responsive to the voltage across the output terminals 0d the rectifier, the output of said another amplifier being electrically connected to the other of the two control windings of said amplifier, whereby the voltage across said other of the two control windings of said amplifier after reaching a predetermined value is equal to the voltage appearing across said one of the two control windings of said amplifier and increases in the same predetermined manner as the voltage across said one of the two control windings, to thereby prevent the voltage across the field winding of the regulated machine from decreasing below the predetermined minimum value.

4. In a system for preventing a voltage regulator from driving the excitation level of a regulated machine below a safe predetermined minimum value, the combination comprising, a field winding for the regulated machine, an amplifier comprising two control windings and disposed to control the energy supplied to the field winding of the regulated machine, a voltage reference network responsive to the output of the regulated machine, the output of the voltage reference network being electrically connected to one of the two control windings of said amplifier to produce thereacross a voltage which increases in a predetermined manner, a sensing network responsive to the voltage across the field winding of the regulated machine and comprising a saturable reactor and a capacitance and inductance network, the saturable reactor comprising a magnetic core member, a control winding disposed in inductive relationship with the core member, the current flow through which is proportional to the voltage appearing across the field winding of the regulated machine, a source of biasing voltage, a biasing winding disposed in inductive relationship with the magnetic core member and responsive to the output of the source of biasing voltage, a rectifier having input and output terminals, one of the input terminals being connected to one side of the ca pacitor and inductance network, a source of aiternating current energy, one side of the source of alternating current energy being connected,

to the other input terminal of the rectifier, the other side of the alternating current source being connected to the other side of the capacitor and inductance network through an impedance device, a main winding disposed in inductive relationship with the core member, circuit means for connecting one side of the main winding to said other of the input terminals of the rectifier, the other side of the main winding being connected to said other side of the capacitor and inductance network, whereby the impedance of said main winding is proportional to the voltage across the field winding of the regulated machine and the voltage across the output terminals of the rectifier being proportional to the impedance of the main winding, another amplifier responsive to the voltage across the output terminals of the rectifier, the output of said another amplifier being electrically connected to the other of the two control windings of said amplifier, whereby the voltage across said other of the two control windings of said amplifier after reaching a predetermined value is equal to the voltage appearing across said one of the two control windings of said amplifier and increases in the same predetermined manner as the voltage appearing across said one of the two control windings of said amplifier to thereby prevent the voltage across the field winding of the regulated machine from decreasing below the predetermined minimum value.

5. In a system for preventing a voltage regulator from driving the excitation level of a regulated machine below a safe predetermined minimum value, the combination comprising, a field winding for the regulated machine, an amplifier comprising two control windings and disposed to control the energy supplied to the field winding of the regulated machine, a voltage reference network responsive to the output of the regulated machine, the output of the voltage reference network being electrically connected to one of the two control windings of said amplifier to produce thereacross a voltage which increases in a predetermined manner, a sensing network responsive to the voltage across the field winding of the regulated machine and which pro duces at its output 'a voltage which increases in a predetermined manner, another amplifier responsive to the output of the sensing network, circuit means including a rectifier for connecting the output of said another amplifier to the other of the two control windings of said amplifier, said another amplifier being biased so that the rectifier prevents currents from flowing from the output of said another amplifier to said other of the two control windings of said amplifier until the output of the sensing network reaches a predetermined value at which time the polarity of the output of said another amplifier reverses and current fiows through said other of the two control windings of said amplifier, whereby the voltage across said other of the two control windings of said amplifier is equal in magnitude to the voltage appearing across said one of the two control windings of said amplifier and increases in the same predetermined manner as the voltage across said one of the two control windings of said amplifier, to thereby prevent the voltage across the field winding of the regulated machine from decreasing below the predetermined minimum value.

6. In a system for preventing a voltage regulator from driving the excitation level of a regulated machine below a safe predetermined min- 12 imum value, the combination comprising, a field winding for the regulated machine, an amplifier comprising two control windings and disposed to control the energy supplied to the field winding of the regulated machine, a voltage reference network responsive to the'output of the regulated machine, the output of the voltage reference network being electrically connected to one of the two control windings of said amplifier to produce thereacross a voltage which increases in a predetermined manner, a sensing network responsive to the voltage across the field winding of the regulated machine and comprising a saturable reactor and a capacitance and inductance network, the saturable reactor comprising a magnetic core member, a, control winding disposed in inductive relationship with the magnetic core member, the current flow through which is proportional to the voltage appearing across the field winding of the regulated machine, a source of biasing voltage, a biasing winding disposed in inductive relationship with the magnetic core member and responsive to the output of said source, a. rectifier having input and output terminals, one of the input terminals being connected to one side of the capacitor and inductance network, a source of alternating current energy, one side of the source of alternating current energy being connected to the other input terminal of the rectifier, the other side of the alternating current source being connected to the other side of the capacitor and inductance network through an impedance device, a main winding disposed in inductive relationship with the magnetic core member, circuit means for connecting one side of the main winding to said other of the input terminals of the rectifier, the other side of the main winding being connected to said other side of the capacitor and inductance network, whereby the impedance of said main winding is proportional to the voltage across the field winding of the regulated machine and the voltage across the output terminals of the rectifier being proportional to the impedance of the main winding, a push-pull magnetic amplifier responsive to the voltage across the output terminals of the rectifier, circuit means including another rectifier for connecting the output of the push-pull magnetic amplifier to the other of the two control windings of said amplifier,

the push-pull magnetic amplifier being biased so that said another rectifier prevents the flow of current to said other of the two control windings until the voltage across said rectifier reaches a predetermined value at which time the polarity of the output voltage of the push-pull magnetic amplifier reverses and current flows to said other of the two control windings, whereby the voltage appearing across said other of the two control windings increases in the same predetermined manner as the voltage across said one of the control windings of said amplifier, to thereby prevent the voltage across the field winding of the regulated machine from decreasing below the predetermined minimum value.

Name Date 2,459,640 Griscom et a1 Jan. 18, 19% 2,608,679 Witzke Aug. 26, 1952 

